Flux leakage detection devices are used for detecting defects below the surface of objects made of ferromagnetic material, such as metal tubing or piping. Flat, curved or otherwise continuous ferrous bodies as well as continuous welds are also susceptible to inspection by this method. A magnetic field is induced in the body being inspected and an array of flux leakage sensors arranged in a predetermined geometry is used to detect changes in the magnetic field near the surface of the body. Discontinuities below the surface of a magnetized material are known to cause variations in the magnetic field above its surface and can therefore be detected by the sensor array. These devices are therefore passed over the surface area of the body being tested in order to locate defects therein. Prior art methods and devices for flux leakage detection have proven useful but leave much room for improvement in multiple areas.
U.S. Pat. No. 4,789,827 describes a magnetic flux leakage inspection unit for use on the inside of pipes and tubes. The device uses flux leakage coils to detect cracks and pits in the wall structure and a Hall Effect sensor to sense variation in wall thickness. Signals from the coils and sensor are fed through an external amplifier to a CRT, magnetic tape or strip chart recorder to generate usable output. The sensing device is limited by the types of objects it can be used to test, the type of data it can output and where it can be used due to the need for external equipment.
U.S. Pat. No. 5,030,911 describes a device for displaying defects in tubular members on a two-dimensional map. Longitudinal and circumferential position detectors are added to an existing inspection unit and the signals from the three devices are fed through an external computer programmed to process the signals and generate data including the longitudinal position, circumferential position, length, and angular orientation of the detected defects. The data can be displayed in both graph and table form using one or more display devices, including a CRT, chart recorder, plotter and/or printer. This testing method is limited by the types of objects it can test, the type of output it can provide and the sensing device's need for external equipment.
U.S. Pat. No. 5,614,825 describes a magnetic flux leakage inspection unit where the sensors are mounted in groups on blocks that are disposed to ride on the surface of the material under inspection. These blocks are mounted with a restricted degree of freedom for movement perpendicular to the test surface so that each individual block can independently respond to surface variations, reducing the degree of variation in the measured field caused by surface undulations and irregularities rather than internal defects. This arrangement is intended to improve accuracy in defect detection, but requires the regular replacement of sensor blocks due to wear caused by having them ride on the surface of the test specimen.
U.S. Pat. No. 5,619,136 describes a magnetic flux leakage inspection unit where a defect is detected when a differential output signal produced a coupled pair of sensors in the sensor array exceeds a threshold value. The device provides a warning signal to the user through LED lights connected to the sensors when a defect is detected, but does not provide for any other useful output data.